Answer 1

The code declares a zero-length array.

int count=0;
int arr[count];

That means no storage is allocated for any array element of arr[]. If it accesses arr[0], then it will go out of bounds of the array and by happenstance it will access the storage for count, but only by a fluke in the way memory is managed. Furthermore, if it accesses arr[1] or arr[2] ... then it will go beyond the memory segment allotted for variable storage. Hence, it reports a memory segment violation exception.

Answer 2

int count=0;
int arr[count];

arr[0]

here your array size is static it wont grow dynamically

Answer 3

By giving arr a length of zero, arr effectively becomes a pointer to the variable that is defined before it. When i=0 it stores n in arr[0], which is the same as writing count = n. That is not the cause of the exception, though.

As i increments to 1, 2, 3,... the code a[i]=n begins overwriting memory outside the bounds of the variable storage and that causes a segmentation fault.